1. Field of the Invention
The present invention relates to an access control circuit for use in an optical disk unit.
The term "optical disk unit" herein includes a magneto-optic disk unit.
2. Description of the Related Art
Recently, there have been developed not only an optical disk unit in which recorded ROM data is reproduced by irradiating a laser beam thereon but also a magneto-optic disk unit in which the user can write desired data into the disk. Such optical disk units are required to have an access control mechanism being small in size and yet capable of seeking out desired data quickly.
The control to locate a particular track in an optical disk unit consists of a slide mode in which the laser beam spot is shifted at a high speed in the radial direction to the target track on the optical disk and a tracking mode in which the beam spot, after being shifted, is kept to follow the target track. When irradiating the beam spot on the track of the optical disk, a laser beam from a light source is focused on the optical disk by means of an objective lens. The slide mode for shifting the beam spot to the target track is generally performed by linearly moving the optical head having the objective lens using a voice coil motor.
The voice coil motor is a linear motor which, when supplied with a drive current, operates such that its moving portion, or slider, makes a linear sliding motion. As the slider is moved, the head coupled with the slider is moved and, as a result, the beam spot is shifted in the radial direction of the optical disk. The number of tracks to be traversed is calculated from the difference between the target track number to be accessed and the current track number and, thereupon, the slider of the voice coil motor is moved such that the beam spot travels the distance corresponding to the calculated number of tracks. At this time, the voice coil motor makes the slide motion in a target speed according to target speeds preprogrammed for numbers of tracks to be traversed.
While the voice coil motor is making a sliding motion, the current sliding speed is detected, and a drive current in accordance with the difference between the detected current sliding speed and the target speed is supplied to the voice coil motor and, thereby, the voice coil motor makes the sliding motion in the target speed. Thus, the speed control in the accessing of the voice coil motor is basically executed such that the voice coil motor is decelerated when it is moving faster than the target speed and accelerated when it is moving slower than the target speed. The conventional access speed control methods include the following three methods.
A first method is such that the driving current supplied to the voice coil motor is changed according to the difference in speed between the current moving speed of the voice coil motor and the target speed. This method will be described below with reference to FIG. 1A and FIG. 1B.
For example, when it is assumed that the current speed of the voice coil motor is faster than the target speed by a speed difference of +s3 in the interval between the times t0 and t1 shown in FIG. 1A, the voice coil motor must be decelerated correspondingly. Therefore, a negative current of -i3 corresponding to the speed difference +s3 is supplied as the driving current to the voice coil motor as shown in FIG. 1B. Thereby, the voice coil motor is decelerated.
On the other hand, when it is assumed that the current speed of the voice coil motor is slower than the target speed by a speed difference of -s2 as indicated in the interval between the times t3 and t4 in FIG. 1A, the voice coil motor must be accelerated correspondingly. Therefore, a positive current of +i2 corresponding to the speed difference -s2 is supplied as the driving current to the voice coil motor as shown in FIG. 1B. Thereby, the voice coil motor is accelerated.
Further, when the current speed of the voice coil motor is equal to the target speed and the speed difference between them is zero as indicated in the interval between the times t4 and t5 in FIG. 1A, the driving current is set to zero as shown in FIG. 1B.
A second method is such that, while the value of the current supplied to the voice coil motor is kept constant, the time period during which the current is supplied is varied according to the speed difference. This method will be described below with reference to FIG. 2A and FIG. 2B.
For example, when the speed difference in the interval between the times t0 and t1 is +s3 as shown in FIG. 2A, the voice coil motor must be decelerated correspondingly. Therefore, a constant negative current of -i3 is supplied to the voice coil motor for a period of time corresponding to the speed difference +s3 as shown in FIG. 2B.
When the speed difference is +s2 smaller than +s3, the time period during which the constant negative current value -i3 is supplied is made shorter, as shown in FIG. 2B, than the time period during which the current was passed when the speed difference was +s3. Namely, control is made such that the time period during which the negative current -i3 is passed through the voice coil motor is made longer the greater the difference in speed for each unit time is. Also, when the speed difference is on the negative side, control is made such that the time period during which a positive current +i3 is passed through the voice coil motor is made longer the larger the speed difference on the negative side for each unit time is.
A third method is that called the BANG-BANG control. In this method, as shown in FIG. 3A and FIG. 3B, a maximum negative current -i3 is supplied to the voice coil motor when its speed is higher than the target speed, while a maximum positive current +i3 is supplied when the speed is lower than the target speed.
Recently, downsizing has come into fashion also in the field of optical disk units and the optical disk unit is tending to become smaller, thinner, and less power consuming. In the conventional voice coil motor, as shown in FIG. 4A and 4B, its moving portion (slider) 2 was supported by roller bearings 8 slidably contacting a pair of guide rails 6. The slider 2 is structured to be integral with an optical head having an objective lens 4. However, as the optical disk unit becomes smaller and thinner, the roller bearings supporting the slider becomes relatively thick. In order to advance the design for a smaller and thinner type, it becomes necessary not to employ a voice coil motor using roller bearings but to employ a voice coil motor, for example, of a slide-along-shaft type.
An optical head employing a voice coil motor of a slide-along-shaft type is schematically shown in FIG. 5A and FIG. 5B. A slider 10 of the voice coil motor is directly and slidably supported by a pair of guide rails 14. The slider 10 is structured to be integral with an optical head having an objective lens 12. As shown in FIG. 5B, the slider 10 is provided with yokes 16 and coils 18, while there is provided magnets 22 in the stator 20 in confronting relationship with the coils 18. The sliding speed and direction of the slider 10 is controlled by the value and direction of the current passed through the coils 18.
When such a slide-along-shaft type voice coil motor is employed, and if the access speed control method described with reference to FIG. 1A and FIG. 1B is used, the voice coil motor becomes suddenly slow or stopped by friction between the shaft and the slider when the drive current becomes small. Thus, there has been a problem that the voice coil motor is difficult to control when it is at a low speed.
Further, in order to realize a drive consuming low power, there is a tendency toward the use of a 5-volt single power source. In this case, since the voltage applied to a current amplifier for supplying the driving current to the voice coil motor is low, the current amplifier operates not in the current mode but in the voltage mode. For example, if the maximum voltage applicable to the current amplifier is 12 V as shown in FIG. 6B, the driving current output from the current amplifier immediately after the application of the voltage 12 V instantly rises to a preset current value capable of driving the voice coil motor as shown in FIG. 6A.
However, when the maximum voltage value applicable to the current amplifier is 5 V as shown in FIG. 7B, the preset current value cannot be reached unless a certain time has elapsed after the voltage 5 V has been applied as shown in FIG. 7A and, hence, the voice coil motor is held inoperative during this time.
Such trouble occurs immediately after the polarity of the driving current has been changed and also occurs when the method of control described with reference to FIG. 2A and FIG. 2B in which a constant current value is supplied for a period of time corresponding to the speed difference or the method of the BANG-BANG control described with reference to FIG. 3A and FIG. 3B is used. When such trouble occurs, it becomes unable to control the voice coil motor to provide a motion at the target speed, and hence quick access control of the beam spot becomes unachievable.